Effect of heat input on failure mode and connection mechanism of parallel micro-gap resistance welding for copper wire

In order to confirm the failure mode and connection mechanism of parallel micro-gap resistance welding with 40 μm copper wire on 300 nm gold plated quartz substrate, the effect of heat input was discussed on bonding interface evolution. Wire bonding processes and their mechanical tests (tensile test and shear test) were carried out to analyze the failure modes. It is shown that the failure modes are bulk separation, partial separation, and melting at middle of interface, which deeply depend on the heat input. Too large a heat input can cause the copper wire and gold layer melt, while too small a heat input can not fully promote element diffusion of the bonding interface and lead to an unreliable connection. Simulation on wire bonding processes was carried out to propose the shape variation of copper wire and temperature distribution of bonding interface. With the increase of heat input, the bonding interface area increases significantly, and the thickness sharply decreases. The connection mechanism of bonding interface is changing from local plastic deformation, subsequent electro-migration to final melting. Meanwhile, the temperature at the bonding interface is exponentially increased, which is deeply related with the generation of intermediate phase. The micro-structure of bonding interface was observed to point out the reaction products. It is shown that the intermediate phases Au₃Cu and AuCu are sequentially formed with the increase of temperature; however, the intermediate phase AuCu is corresponded with better connection performance.